Harden fm-shim-backend URI length handling

[assisted-by-ai]

Motivation

• Prevent signed-to-unsigned conversion issues and oversized allocations when handling URI lists from D-Bus which could lead to memory-safety problems.

Description

• Changed launch_frontend_process signature to accept size_t uri_list_len so allocation/index math is size-safe.
• Added an explicit overflow guard if (uri_list_len > SIZE_MAX - 4) before allocating the argument array for execve().
• Validated the int uri_list_len returned by dbus_message_get_args to detect and reject negative lengths before converting to size_t.
• Propagated the validated size_t length into both the no-reply and normal reply code paths when calling launch_frontend_process.

Testing

• Ran syntax/quick checks with gcc -fsyntax-only and gcc -x c -fsyntax-only to validate the changes; these checks failed to complete a full build because the container lacks dbus-1/libsystemd development headers and pkg-config metadata, so a full compile/link test could not be performed.
• No unit tests are present; changes were kept minimal and focused on type/overflow checking to limit behavioral impact.

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Codex Task

[assisted-by-ai]

Summary
Reviewed fm-shim-backend.c for memory-safety issues around URI count handling and fixed an unsafe signed→unsigned path by changing launch_frontend_process() to accept size_t for URI list length. This removes repeated casts and keeps indexing/allocation arithmetic type-safe.

Added an explicit overflow guard before building arg_arr (uri_list_len + 4) to prevent wraparound in allocation sizing for execve() arguments.

Added validation for negative uri_list_len returned from D-Bus argument decoding; invalid negative lengths are now rejected before conversion, preventing huge size_t values and potential oversized allocations/out-of-bounds behavior.

Updated both call paths to pass the validated size_t length into launch_frontend_process() consistently (no-reply and normal reply flow).

[ArrayBolt3]

Integrated the useful changes from this in ArrayBolt3@c96fb11.